Panel size and dimensional stability scenario

[Adam Aron]

Baseline scenario: 3x 3/4" alder boards, 5" wide, and 24" long glued up to become 15" x 24"

Hypothetical scenario (to gain dimensional stability): 5x the same 3/4" alder boards, but 3" wide, and again 24" long glued up to become the same 15" x 24" size.


What is your opinion on how much, if any, dimensional stability gain to expect from the hypothetical construction?

In other words, when encountering humidity changes, will this construction model offer any significant protection against warping, cupping, etc.?

I know there are lots of theories, but I'm hoping someone can save me the extra work of gluing up 5 boards instead of 3 if it isn't worthwhile in the long run.

Thanks in advance.

[Jason Beam]

It's been my experience (not just theory or research) that it depends heavily on the boards themselves. Are they properly dried? Are they flat sawn? Quarter? Rift? Do they have figure? Do they contain any extra tension (as would a board with knots or one grown near limbs)?

There are so many variables you can't control that I say glue it up as you have it in whatever dimension yields the best looking panel. If it's properly dried, you should be okay most of the time - allowing for the chance that some of the previously mentioned factors to change that risk.

I've done glue-ups with 2" wide sticks. Stayed stable, just fine. I've done glue-ups with 11-1/2" wide boards, stayed stable. It depends on the boards, how acclimated they are, what the environment is, etc.

All this to say: do what looks best. You never really know if it's gonna warp until it does.

[Adam Aron]

It's been my experience (not just theory or research) that it depends heavily on the boards themselves. Are they properly dried? Are they flat sawn? Quarter? Rift? Do they have figure? Do they contain any extra tension (as would a board with knots or one grown near limbs)?

Hi Jason, thanks for your response. The boards are relatively clear alder, flat sawn, without much figure. They seem to be well dried and they've been in my garage/shop for a couple weeks.

Another part of the puzzle that I didn't mention is that it doesn't matter what looks good because most of will be veneered (both sides). But, the veneer will be cut in some parts in the middle and also trimmed on the edges to reveal some of the alder boards. If it sounds weird, that's because its more of an "art" piece.

Does any of that change your bottom line advice?

[Jason Beam]

Not even a little, honestly. If anything, veneering both sides is going to do more for stabilizing the panel than the dimensions of the boards in the core.

[Shawn Pixley]

I'd say unless I saw something odd with the 5" wide boards, I'd go with gluing (3) 5" widths. If I looked at the end grain and saw severe curve in the growth rings across the 5" (meaning it was from a smaller log), I might goto the (5) 3" pieces.

[Jim Matthews]

What's the application?

I think you'll need a backing veneer, to ensure stability over the long haul.

As mentioned above, veneer with a finish will form something
of a moisture barrier. If the end grain of the panels are also finished,
it's not likely to move much across the thickness of the board.

The width is another matter entirely.

Proper veneering is demonstrated in plywood,
where there is a substrate with grain oriented 90 degrees
to the 'show' face. This makes for a stable panel.

I would suggest that your concern about the stability of the substrate is
well-founded. Furniture that is surface veneered directly over a hardcore
shows cracks along the grain, where the veneer was split by hydostatic forces.



http://engineeringtraining.tpub.com/.../14069_199.htm

[Adam Aron]

I'd say unless I saw something odd with the 5" wide boards, I'd go with gluing (3) 5" widths. If I looked at the end grain and saw severe curve in the growth rings across the 5" (meaning it was from a smaller log), I might goto the (5) 3" pieces.

Here's the glue up I finished yesterday. I got the end grain wet to show up in the photo. Do you see the growth rings being a problem for me? Alder isn't typically in very large logs, so I think these are fairly typical. I chose alder wood in large part for its relatively good stability.

3alderboards.jpg

The end grain will be finished because it will be routered with a cove bit on all sides. I haven't decided on the finish yet. I still need to research the moisture barrier effects of different finishes.

Thanks again.

[Adam Aron]

As mentioned above, veneer with a finish will form something
of a moisture barrier. If the end grain of the panels are also finished,
it's not likely to move much across the thickness of the board.

The width is another matter entirely.

Proper veneering is demonstrated in plywood,
where there is a substrate with grain oriented 90 degrees
to the 'show' face. This makes for a stable panel.

I would suggest that your concern about the stability of the substrate is
well-founded. Furniture that is surface veneered directly over a hardcore
shows cracks along the grain, where the veneer was split by hydostatic forces.

Hello Jim, it seems you are saying I need to be more concerned about perpendicular grain in boards vs veneer. So, in this setup, a veneer with grain running across the width would be the only way to go for stability?

Conversely, (considering the photo I posted for Shawn) do you think I would be better off with 5x ~5" wide boards that are 15" long to get my final glue up size? That is, if the veneer grain runs along the 24" dimension.

Much appreciated. Learning a lot!

[Adam Aron]

What's the application?

It will end up hanging on the wall with a mirror in it. I'm trying to find out if there's any way putting a mirror into a 3/8" deep mortised area in the middle will work, or if the mirror will be doomed to break. I'm looking for a way to glue it with a cushion-like material behind it to absorb any warp (like a netting). Or maybe it can't be glued at all, only affixed with framers points or something similar. I was thinking mortising out the middle instead of cutting a hole will retain wood and therefore maintain stability.

[Shawn Pixley]

Here's the glue up I finished yesterday. I got the end grain wet to show up in the photo. Do you see the growth rings being a problem for me? Alder isn't typically in very large logs, so I think these are fairly typical. I chose alder wood in large part for its relatively good stability.

3alderboards.jpg

The end grain will be finished because it will be routered with a cove bit on all sides. I haven't decided on the finish yet. I still need to research the moisture barrier effects of different finishes.

Thanks again.

Adam,
It still might not be a problem, but when I look at that end grain you can see crests above one face and has both ends below the other face. The log was fairly small. The outer rings will expand more than the inner rings making it prone to cupping. As you show it the growth rings are reversed, so that will tend to equalize it somewhat.

My statement above is for the 5' width plain sawn above. If the boards were wider, then it is inevitable that the rings would crest like that. That amount of curve on a small board would give me some pause.

I have glued much wider boards together without problem, so there is no hard and fast rule.

[Shawn Pixley]

The mirror should be stopped in rather than glued to a substrate. Why not make a frame for the mirror rather than a panel?

[Howard Acheson]

There is basically no change in stability when using using multiple boards glued up as a panel.

In other words, gluing up three 4" wide boards will react the same as gluing 4 3" wide boards. providing you are working with the same species of wood. Both glued up panel situations will expand and contract the same amount.